Meaning of Friyay
Friday is a mix between Friday and yay. People say Friday to express celebration and joy as the weekend arrives. This word is used to express pleasure about Friday, although many people use this word for other days as well, such as Saturday, Thursday, etc. but not Monday for apparent reasons.
Why do people use this slang?
This word was originated by people on social media to indicate excitement about Friday. People like celebrating this day as they work 5 days a week and they feel relaxed when the week is over and the weekend comes. Hence, they have combined Friday with yay.